1. Field of the Invention
The subject invention relates to methods and apparatus for guiding a tape and, for instance, has utility in magnetic tape transports, photographic film handling apparatus and other tape or web transport or handling equipment wherein precision guidance of a tape, film or other web is required or desired. The subject invention relates also to vacuum columns and to tape transports employing vacuum columns.
2. Disclosure Statement
This disclosure statement is made pursuant to the duty of disclosure imposed by law and formulated in 37 CFR 1.56 (a). No representation is hereby made that information thus disclosed in fact constitutes prior-art inasmuch as 37 CFR 1.56(a) relies on a materiality concept which depends on uncertain and inevitably subjective elements of substantial likelihood and reasonableness, and inasmuch as a growing attitude appears to require citation of material which might lead to a discovery of pertinent material though not necessarily being of itself pertinent.
Throughout the years, many precision guiding techniques and apparatus have been proposed or implemented. By way of example, and not by way of limitation, precision guiding of a tape or web is important in information processing or handling equipment wherein a tape or web shaped information carrier needs to be accurately positioned relative to information recording and playback devices.
A drawback of conventional guiding techniques customarily resides in an imposition of uneven stresses across the tape surface. For instance, U.S. Pat. No. 2,862,715, by D. N. MacDonald, issued Dec. 2, 1958, proposes the use of several tape guides, each having a tapered body bounded by a reference surface extending at right angles to a longitudinal axis through the tapered body. The tapered body, which acts as a tape guide, imposes on the advancing tape a transverse force component by stressing the advancing tape unevenly across its major tape surface. In particular, that tapered guide stresses one edge of the advancing tape more than the other, thereby loading the tape against the straight reference shoulder provided at the base of the tapered body. This, in turn, has the drawback of prompting the tape to ride up or curl up on the reference shoulder, thereby adding further wear and tear to the wear and tear already occasioned by the uneven stresses imposed on the tape by the tapered guide.
A further type of web guide is apparent from U.S. Pat. No. 3,281,040, by F. F. Grant, issued Oct. 25, 1966. According to that proposal, a positive pressure gradient is established at the guiding element so that the moving web tends to slide "downhill" of the gradient and the edge of the web adjacent a guiding shoulder is brought into contact with that shoulder. Web guides of this type are useful in some applications but tend to be relatively long, bulky and air consumptive.
Further tape guide systems are apparent from U.S. Pat. No. 4,091,979, by L. B. Browder, issued May 30, 1978 to the subject assignee. While those systems afford a very high tape guiding precision, they would share with conventional approaches the disadvantage of tape curlup, if they employ a straight reference shoulder extending at right angles to the tape guiding surface. Such systems are further vulnerable to the effects of tape width variation or non-straightness of tape edges. Even with high-quality tape, variations of the order of 0.002/0.003 inches (50-75.mu.) are relatively frequent and result in a substantial variation of the force between the tape edge and adjacent reference shoulder, eventuating curling of the tape edge and climbing thereof onto the reference surface.
An excellent tape guide with one or more sloping reference surfaces is apparent from copending U.S. patent application Ser. No. 58,225, filed July 16, 1979 by F. F. Grant and H. M. Martija, and assigned to the subject assignee.
In its embodiment, that tape guiding system employs a tiltable roller for imposing a yaw motion on the advancing tape, applying same to the sloped reference surface or surfaces.
Reference may also be had at this juncture to the contents of the disclosure statement of the latter copending patent application, as far as further prior proposals are concerned.
A persistent problem with even the highly advanced systems so far discussed stems from the fact that an imposition of unequal radii at the two tape edges within a provided bend of the tape is inherent therein.
This applies also to a past proposal to slant the sides of a vacuum column so as to urge the tape traveling in a loop through the vacuum column against a reference surface. In particular, the slanted sides of the vacuum column in that proposal necessarily impose unequal radii on the two tape edges of the bight portion of the U-shaped tape loop in the vacuum column.
In brief, there exists a need for tape guiding methods and apparatus which are characterized by an inherent provision and preservation of equal radii at both edges of a tape advancing through a requisite arcuate bend.